We have shown that T cells from two strains of mice congenic at the major histocompatibility complex (MHC) use different T cell receptor (TCR) gene elements to encode receptors recognizing very similar antigen/MHC determinants. Possible explanations for this strain dependent TCR gene element usage include 1. a different starting pool of germline TCR gene elements from which functional genes can be derived, 2. a skewing of the peripheral TCR repertoire by in vivo presentation of antigen, 3. a skewing of the TCR repertoire due to the imposition of self tolerance, and/or 4. a skewing of the functional TCR repertoire by the positive selection for self MHC restriction specificity. The first possibility has been ruled out, and we will use two approaches to identify which combination of the remaining possibilities forms the basis for this unusually clear strain dependency of TCR gene element usage. The first approach involves the analysis of TCR expression in antigen specific T cells that differ from one another in the environments in which they have differentiated and met antigen. If A and B represent the two congenic strains of mice that differ in their TCR repertoires, we will analyze the TCR gene elements used by T cells exhibiting indistinguishable antigen specificities from (F1(AxB)-->A) and (F1-->B) radiation chimeras. In both sets of animals, the antigen presentation cells are of F1 origin and the T cells are tolerant of F1 antigens, but the same pool of F1 stem cells has developed either into an MHCa restricted population of mature T cells on the one hand, or an MHCb restricted population on the other. We will also analyze the TCR expression in F1 T cells that have been primed in a parental environment. The combination of these studies will allow us to determine whether the relationship between MHC expression and the TCR repertoire of cells with the same functional phenotype lies at the level of antigen presentation, induction of self tolerance, selection of MHC restriction specificity, or some combination of these factors. As a second approach, we will engineer transgenic strains of mice to directly assay the influence of the MHC on the level of expression of these strain dependent TCR gene elements. Embryos will be injected with a construct of DNA encoding a complete, rearranged TCR gene expressed mainly in B strain mice, or a construct including both types of gene elements in an unrearranged configuration. Lines of mice transgenic for these constructs will be backcrossed to A strain and B strain mice and screened for the presence of the transgene and the exclusive expression of MHCa or MHCb antigens. We will determine the influence of the MHC on the level and type of transgene expression in various subpopulations of naive and antigen primed T cell populations.